1. Field of the Invention
The present invention concerns a method to detect position changes of a medical wherein x-ray detectable markers are arranged in the environment of the implant, and wherein a positional change of the implant is checked using 2D x-ray exposures at temporal intervals of a region containing the implant, using a distribution of the markers in each 2D x-ray exposure, with a first of the 2D x-ray exposures being obtained at a first point in time from one projection direction, and a second of the 2D x-ray exposures being obtained at a second point in time from another projection direction. The invention also concerns an x-ray system fashioned for implementation of the method.
2. Description of the Prior Art
Supervision of the change of the position of implants, in particular the position of implanted prostheses, is an important post-operative measure in order to be able to undertake timely corrections and to prevent complications. Immediately after the implantation of artificial joints, the prosthesis can loosen in the course of time due to weakening of the materials used. In order to recognize this loosening, in the implantation, in addition to the prosthesis, small metal spheres that can be clearly localized in x-ray exposures of this region are placed in the bones bordering the prosthesis. A shifting of the artificial joint relative to the bones can be detected in the x-ray exposure with these metal spheres serving as markers. Such a shifting is an indicator of the loosening of the artificial joint.
A known method for detection of the position change of an implant (as it is described, for example, in Selvik, G., Roentgen stereophotogrammetry, Acta Orthopaedica Scandinavica Supplementum No. 232, Volume 60, 1989, Reprint from the original 1971 thesis) requires the implementation of at least four x-ray exposures with a calibration pattern in order to be able to detect corresponding position changes. For this, at each of regular temporal intervals two 2D x-ray exposures of a region containing the implant are obtained together with the calibration pattern from two projection directions predetermined by the calibration pattern. Corresponding spheres as well as marked points of the implant in the exposures are respectively, manually marked in the 2D x-ray exposures acquired at each interval. With the help of these correspondences, the position of the spheres and marked points in the 2D x-ray exposures and the measurement data of the calibration pattern, a 3D model of the spheres in the bones as well as of the marked points of the implant is generated. The comparison of the 3D model calculated in this manner at each interval with a 3D model calculated earlier or later enables a decision as to whether a position change of the implant has occurred, and what dimension this position change exhibits. The comparison requires the consistent numbering of the individual points or spheres in the 2D x-ray exposures. The method is tedious and additionally stresses the patient with the x-ray dose applied in the x-ray exposures.